Crop Sensors for Nitrogen Management

This article says that crop sensors can help in making nitrogen application decision on growing crops.  There is no mention of using PSNT testing to calibrate the sensors, but it seems like a good way to go.  Sensors have been used successfully often enough that they should be considered.  The sensors in his article are chlorophyll sensors.  Soil color sensors have also been experimented with.  The theory on soil color sensors is that they will sense darker and lighter colored soils.  Dark soils tend to be higher in nitrogen and therefor have more potential to release nitrogen.